This invention relates to a digital signal transmission system, and more particularly to a serial signal transmission system.
For instance, in a split type air conditioning apparatus, data required for the operation of the apparatus are transmitted between an indoor unit and an out-door unit of the apparatus. Since the amount of data to be transmitted in this case is not much, and the economization in the number of signal lines is mandatory, serial signal transmission systems as shown in FIGS. 1 and 2 have been heretofore proposed.
In the system shown in FIG. 1, signal transmission lines L.sub.3 and L.sub.4 are provided separately from AC power supply lines L.sub.1 and L.sub.2 between a transmitting circuit and a receiving circuit of the system. Serial signals generated in a serial signal generating device 1 in the transmitting circuit of the system are transmitted from a serial signal transmitting device 3 also provided in the transmitting circuit through the signal transmission lines L.sub.3 and L.sub.4 to a serial signal receiving device 4 and a serial signal discriminating device 2 both provided in the receiving circuit.
On the other hand, in the system shown in FIG. 2, the power supply lines L.sub.1 and L.sub.2 are utilized for transmitting signals. That is, the serial signals generated in the serial signal generating device 1 in the system shown in FIG. 2 are modulated, for instance, in the serial signal transmitting device 3, and then transmitted through the power supply lines L.sub.1 and L.sub.2 to the serial signal receiving device 4 to be demodulated therein.
Although the circuit shown in FIG. 2 is advantageous because the signal transmission lines L.sub.3 and L.sub.4 are omitted, it requires complicated modulation and demodulation circuits.
For eliminating this disadvantage, there has been proposed another system as shown in FIG. 3 wherein a signal transmission line L.sub.3 is provided between the signal transmitting circuit and the signal receiving circuit of the system, and each of the positive half cycles of the power source voltage is utilized as one bit of the serial signals to be transmitted from the transmitting circuit to the receiving circuit of the system.
Although the system shown in FIG. 3 is advantageous in that it requires only a single signal transmission line without requiring modulation and demodulation of the signals, the instants at which the serial signal generating device 1 of this system generates serial signals are those when one of the power source lines (L.sub.1 in the example shown) connected to the device 1 becomes positive, and therefore any erroneous connection of the device 1 to the power source line has rendered the system utterly inoperative.